An asynchronous operation cannot be started at this time. Asynchronous operations may only be started within an asynchronous handler or module or during certain events in the Page lifecycle.

An asynchronous operation cannot be started at thistime. 
Asynchronous operations may only be started within an asynchronoushandler 
or module or during certain events in the Page lifecycle. If this exception occurred while executing a Page, 
ensure that the Page is marked<%@ Page Async="true" %>

Description: Anunhandled exception occurred during the execution of the current web request.
Please review the stack trace for more information about the error and where itoriginated in the code. 

Exception Details: System.InvalidOperationException:An asynchronous operation
 cannot be started at this time. Asynchronousoperations may only be started within an asynchronous
 handler or module orduring certain events in the Page lifecycle. 
If this exception occurred whileexecuting a Page, ensure that the Page is 
marked <%@ PageAsync="true" %>.

I got the following error while running the application; we needed to add Async attribute to the page directive and set the value is set to true.

<%@ Page Language="C#"  AutoEventWireup="true" Async="true" CodeBehind="ProgressForm.aspx.cs" Inherits="MVC_tutorials.ProgressForm" %>